Control system



Feb. 23, E E MOYER r v CONTROL SYSTEM Filed April 25, 1942 Inven her 5 E mo E. M05 e1" Orr-in W. Livingsoon, Hewr- H Laugh I by 7% 5% Th air AttoT-Tl e Patented Feb. 23, 1943 UNITED STATES PATENT OFFICE and Henry H. Leigh, Schenectady N. Y., as-

signors to General Electric Company, a corporation of New York Application April 23, 1942, Serial No. 440,158

16 Claims.

This invention relates to control systems, more particularly to systems for controlling the operation of electric motors, and it has for an object the provision of a simple, reliable, and improved control system of this character.

More specifically, the invention relates to systems for controlling the starting of an electric motor, and a further object of the invention is to accelerate the motor at the maximum permissible rate to a desired speed which may be preset upon a calibrated speed control device.

Another object of the invention is the provision of a control system in which the current is limited to a predetermined value therebyto accelerate the motor to the preset speed without QXCQSSWB current peaks.

Another object is to compensate for the internal voltage drop of the armature of the motor during acceleration and. normal running operation of the motor.

A further object of the invention is the provision of a wide, stepless range of operating speeds.

. A still further object of the invention is the provision of a compact variable speed motor drive supplied from a source of alternating current in which gears, pulleys, clutches, and bulky apparatus heretofore used are eliminated and in which the objects set forth in the foregoing are achieved largely by the use of inexpensive electronic devices.

In carrying the invention into effect in one form thereof, a direct current motor is supplied through a transformer and suitable electric valve apparatus from a source of alternating voltage. In order to preset a desired operating speed for the motor, a movable control device which may be calibrated in terms of speed produces a reference voltage corresponding to a desired operating speed, and a control voltage dependent upon the speed of the motor is derived from the motor. The difference of these two voltages is utilized to control the electric valve apparatus to accelerate the motor to the desired preset operating speed, or to vary the speed of the motor from one value to another as desired. In addition, means responsive to the armature current or to a quantity proportional thereto, superimposes a controlling effect on the electric valve apparatus to limit the current supplied to the ar-, mature to a predetermined maximum value thereby to provide for accelerating the motor to a desired speed in a minimum of time and without any current peaks.

For a better and more complete understandat ing of the invention, reference should now be had to the following specification and to the accompanying drawing, the single figure of which is a simple, diagrammatical representation of an embodiment of the invention.

Referring now to the drawing, an electric motor l0 having an armature Illa and a shunt field winding Hlb is supplied from a source of alternating voltage ll through a supply transformer l2 and suitable electric valve apparatus comprising electric valves l3, l4, l5, and IS. The starting, stopping, and reversing of the motor l 0 are under the control of a suitable controlling accessory such as a push button station l1 and a pair of feed controlling rheostats l8 and I 9. If desired, the speed controlling rheostats l8 and I9 may be mounted on the push button station control panel.

The secondary winding of the supply transformer I2 is provided with a midtap We and this midtap is connected to the bus 20 which becomes the negative side of the direct current system, i. e., the negative armature terminal, the negative field terminal, and the negative control terminal. Assuming that the motor to is designed for operation on 250 volts, the full secondary voltage of the transformer will be approximately 620 volts. Two 57 -volt taps MD and I20 either side of center provide a l15-volt tapped source of control power for filaments, transformers, phase shift bridges, and relay excitation.

Suitable non-linear resistance units 2| and 22 are connected across each half of the secondary winding. These non-linear resistances may be made of any suitable material having a nonlinear characteristic but are preferably made of a composition of silicon carbide crystals which are held together by a suitable binder. Resistance material of this character is sold on the market under the trade-mark Thyrite and is described and claimed in United States Patent 1,822,742-Karl B. McEachron, dated September 8, 1931. The purpose of these non-linear resistances is to protect the inductive winding from high voltage surges which sometimes occur in tube circuit operations. The non-linear resistance provides a low resistance shunt path to voltages several times higher than normal, and a very high resistance path to normal voltage.

The current supplied to the field winding lib of the motor from the source II is controlled by electric valves 13 and it. As shown, the anodes Na and 16a of electric valves II and it are connected by means of conductors 23 and 24 to opposite terminals of the secondary .winding' of means of conductors armature the anode circuits.

transformer I2. The filamentary cathodes I3?) and ISb are heated by means of current supplied to the cathodes through a filament transformer 25, the primary winding of which is connected to the 1l5-volt taps I2b and I20 of the supply transformer, and the secondary windings 25a and 25b of which are connected to the filamentary cathodes I31) and IN), respectively. These secondary windings 25a and 25b of the filament transformer are midtapped, and the midtaps are connected to the conductor 23 which thus becomes the positive terminal of the field supply. Thus, the field circuit may-be traced from the positive conductor 26 through a conductor 21, the operating winding of a field protective relay 28, the contacts 29a of a time delay relay 29 through field winding IN to the negative conductor 20. The supply of current from thesource II to the armature IIla of the motor is controlled by means of the electric valves I4 and I5." As shown, the electric valves I4 and I5, like the electric valves I3 and I6, are connected for bi-phase rectification, 1. e., their anodes Ila and Ia are connected through contacts tor 30 and through primary windings 3Ia and 3Ib of a special control transformer 3I to the opposite terminals of the secondary winding of the supply transformer I2. The cathodes Ilb and IN) of valves I4 and I5 are provided with suitable heating units which are connected to the secondary windings 25c and 25d of the filament transformer 25. The cathodes Mb and I517 are connected by 32 and 33, respectively, to thus becomes the positive side of the supply for the armature. .Thus the circuit is readilytraced from the positive. terminal 34 through conductor'35, the heating element 36a of an overload protective relay 36, one or the other of the directional contactors 31 and 38', and then through armature Ilia to the negative conductor 20. If desired, a smoothing reactor (not shown) may be connected in the positive direct current motor armature lead 35.

Although the electric valves I3, I4, I5, and I6 may be of any suitable type, they are preferably grid controlled, mercury vapor thyratron tubes. The cathodes Nb and I5b of the valves I4 and I5 which control the supply of current to the armature are indirectly heated, and these valves are provided with shield grids I40 and I5c as well as with control grids Md and I5d, respectively. The valves I3 and I6 which control the supply of current to the field winding have directly heated filamentary cathodes and have only single grids I30 and I60 which are the control grids. In thyratron valves, the function of the control grid is only to initiate the now of current between the anode and cathode during each positive halfcycle of anode voltage. Once current has started to flow, the grid exercises no further control until the conductivity .of the valve has been interrupted by some means external to the valve itself. Once the current has ceased to 110W, the potentiaiof the grid will again determine the point in the positive half-cycle of anode voltage at which the valve will again become conducting. Thesevalves are therefore grid controlled arc rectifiers.

The purpose of the time delay relay 29 is to allow time for the initial heating of the cathodes of the electric valves beforepower is applied to Since normally open condelay'relay 29 are included in theenergizing circuit of the field protective relay 28, which has normally open contacts 28a in the energizing circuit of the directional conthe terminal 34 which tacts 29a of the time 30a and 30b of anode contactactors, which, in turn, have normally open interlock contacts in the energizing circuit of the anode contactor 30, the anode contactor cannot be closed to apply voltage to the anodes of the electric valves until a predetermined interval after the connection of the supply transformer I2 to the source of alternating voltage II.

The purpose of the field protective relay'28 is to delay the possibility of the application of 10 voltage to the armature of the motor until a safe field excitation has been established and to interrupt the armature circuit in case of a field failure.

Although the thyratron valves I3, I4, I5 and I8 may be controlled by any suitable method, it is preferred to use the-method of phase shift control of the grid voltage. For the carrying out of this method of control, a pair of phase shiftin networks, one for the armature thyratrons and one for the field thyratrons, are provided. The

phase shifting network for the armature thyratrons comprises a resistor 39 and the alternating current winding 40a of a saturable core type reactor 40, and the network for the field thyratrons comprises a resistor 4| and the alternating current winding 42a of a, saturable core type reactor 42. The network for the field thyratrons is connected across the low voltage terminals I2b, I2c through the interlock contacts 361) of the overload relay 36. Similarly, the network for the armature thyratrons is'connected across the low voltage terminals I2!) and I20, but this circuit passes through interlocks On the reversing contactors 31 and 38, through contacts of the F,Rand Stoppush 35 buttons on the push button control panel I'I,

through the field protective relay contacts 28a, and

through the overload relay contacts 36b. The

primary winding 43a of a grid transformer 43 is connected between the midtap I2a and the junction point 39a of the resistor 39 and reactor winding 40a. This grid transformer has two secondary windings 43b and 43c. The secondary winding 43b is connected between the cathode and grid of the armature thyratron I4 and similarly, the secondary winding 43c is connected between the cathode and grid of the armature thyratron I5. A corresponding grid transformer 44 is provided for the field thyratrons I3 and I6 which has arprimary winding a connected between the midtap I2a and the junction point 4Ia of the resistor 4| and reactance winding 42a, and a pair of secondary windings 44b and c which are connected between the cathode and grid of the thyratrons I3 and I6, respectively. The phase shift of the grid voltages is' produced by varying the reactance of the saturable core reactors, which is controlled by varying the DC saturation of these reactors.

The control is such that when the saturable reactors are saturated, the voltages of the grid transformers tend to be in phas with the anode transformer voltage, and when the reactors are unsaturated, the voltages of the grid transformers tend to be out of phase and lagging. Intermediate values of saturation produce intermediate phase relationships. Thus, when the saturable reactors to and 42 are fully saturated, the thyratrons I3, I4, l5, and I6 are fully conducting and conversely, when the reactors are unsaturated, the thyratrons are non-conducting. For intermediate values of saturation, the thyratrons have corresponding intermediate values. of conductivity.

The push button station I1 is provided with 75 a plurality of push button type switches l5, 4B,

and 41 for controlling the starting, stopping. and the direction of rotation of the motor l5. The push button switch 45 controls the starting of the motor in the forward direction; the push buttonv switch 45 controls the starting or the motor in the reverse direction; and the push button switch 41 controls the stoppin oi the motor.

To the same anode transformer terminals as those to which the anodes of the thyratrons are connected, there is also connected a small auxiliary rectifier valve 48 which furnishes a separate source of direct voltage from which the control electric valves are energized. The saturating windings of the saturable reactors 45 and 42 obtain their energization from this source of direct voltage. This direct voltage is filtered by means of a smoothing reactor 55 and a capacitor The voltage across the capacitor 5| is impressed on acircuit comprising a resistance 52b in series with two glow tubes 53 and 54. These glow tubes 53 and 54 are gaseous discharge devices which operate in that region of their characteristic in which the voltage drop across the tube is substantially constant over a wide range of current. The voltage across the points 52a and 54a is fixed in magnitude by the type of glow tube used, and within the operating limits or this equipment, this voltage is independent of variations in the alternating current supply voltage. Any difference in voltage between the voltage drop across the capacitor 5| and the constant voltage across glow tubes 53 and 54 is absorbed by the resistor 52b.

The voltage across the glow tubes 53 is used to stabilize the voltage on the amplifier valves which are connected between the points 52a and 53a. The voltage drop across the glow tube 54 is the voltage standard with which signal voltages are compared for controlling purposes.

For the purpose of varying the direct current which flows in the saturating winding 45a, a suitable amplifying electric valve 55 is provided. This valve is provided with an anode 55a, a cathode 55b, and a control grid 550. It will be noted that the direct current winding of saturable reactor 45 and the valve 55 are connected in series across the tube 53. The control 01' the current through the direct current winding of the armature saturable reactor 45 is achieved by proper choice or the grid-to-cathode voltag of the valve 55. As the voltage of the grid 550 is made less negative with respect to the voltage of the cathode 552), the current transmitted by the valve will increase, thereby increasing the saturation of the armature saturable reactor 45 which, as pointed out in the foregoing, results in increasing the voltage applied to the armature of the motor l5. Conversely, as the voltage of the grid 550 is made more negative with respect to the voltage of the cathode 55b, the current transmitted by the valve will decrease and this will decrease the voltage supplied to the armature oi the motor. An additional aniplii'ying electric valve 55, which is provided with an anode 550, a cathode 55b, and a control grid 550, is provided for the purpose oi varying the voltage on the grid 550 so that the speed of the motor I5 is maintained constant at a preset value which is correlated with the position of the slider |5a on the speed controlling potentiometer H. In other words, the electric valve 55 serves as a connecting link between the armature speed controlling potentiometer I5 and the electric valve 55 which controls the saturation 75 of the armature saturable reactor 45. and hence. controls the armature voltage and speed of the motor '15. The grid 55c oi. valve 55 is connected to a voltage divider comprising resistors 5 51a, 51b, and lie. The electric valve 55 is connected between the slider Ila or the speed control potentiometer i5 and the Junction point oi the resistors 51a and 515. when the voltage or the grid 55c is made less negative with respect to its cathode, the current transmitted by the valve 55 is correspondingly increased, and since this current flows through the resistor 51a, the voltage drop across this resistor is. correspondingly increased and consequently, the voltage oi the grid 550 is correspondingly decreased so that the current transmitted by valve 55 is decreased and the armature voltage and speed are correspondingly decreased. Thus, increasing the conductivity of electric valve 55 has the effect of decreasing the voltage supplied to the armature of motor l5 and conversely, decreasing the current transmitted by electric valve 55 has the effect of increasing the voltage supplied to the armature of motor 15.

5 Since the lower terminal of resistor 51c and one electrode of tube 54 are connected together at point 54a, and since the upper terminal of resistor 510 is connected to the grid 55c, and the upper electrode of valve 54 is connected to the cathode 55b, the valve 55 compares the voltage drop across the resistor 510 with the voltage drop across the tube 54.

If the armature voltage or a portion of the armature voltage is impressed on the grid 55c,

an increase in armature voltage will increase the conductivity of valve 55 thereby decreasing the conductivity of valve 55 and decreasing the output of thyratrons I4 and I5 and thereby correcting the increase in armature voltage. It the armature voltage decreases, the reverse action takes place and the decrease in armature voltage is corrected. The position of the slider Ilia on the armature speed control potentiometer determines the percentage of the total voltage drop across the valve 54 which is to be derived and used as a preset indication of speed. The voltage that is so derived and used as a reference voltage is the voltage between the slider and the negative bus 25. Since the cathode of the valve 55 is connected to the slider, then the position of the slider will determine the voltage of the cathode relative to the negative bus 25. A signal voltage is derived from the armature voltage by means of a voltage divider which comprises resistors 58a, 58b and that portion of the resistor 55 between the slider 55a and the negative bus 25. The signal voltage used is the voltage from the junction point 01' the resistors 58a and 55b to the negative bus 25, and this signal voltage is impressed on the grid 550 of valve 55. Thus the grid-to-cathode voltage of the valve 55 is the difference between the signal voltage and the voltage from the slider I to to the negative bus 25. The tendency of the circuit will be to maintain the signal voltageapproximately equal to the reference voltage, i. e., the voltage from the slider I80 to the negative bus 25. Hence, the armature voltage and the speed of the motor will be approximately proportional to the reference voltage tapped off by the slider Us 0! the speed control potentiometer.

The voltage selected by the position of the 'z .position'fi, the

' For the purpose of varying the saturating current of the field saturable reactor 42, a pair of electric valves 6| and I corresponding, respectively, in function to the valves 55 and B 0! the armature control, is provided.- The electric valve 6| has an anode a, a cathode 60b, and a control grid "c, and similarly the valve 6| has an anode Ila, a cathode tlb, and a control grid llc. In the control of the field current, the voltage across the field winding lllb is assumed to be an indication of field current, i. e., it is assumed to be proportional to field current. The connections and operation of the valves 80 and ii are similar to the connections and operation of the valves 55 and 56 of the armature control. The electric valves 6! and H operate to compare the voltage across the field winding lb, or a selectable portion thereof, with a reference voltage which is derived from the voltage of glow tube M by means of the slider position on the field weakening potentiometer I! which is connected in series with a potentiometer 62 across the glow tube 54. The diflerence between the signal voltage derived from the field winding and the reierenceyoltage derived from the tube 54 is impressed on the grid-cathode circuit of the valve 6| in such a manner that if the field volt age increases, the conductivity of the valve 8| increases, thereby decreasing the conductivity of the valve 60 and the saturation of the field saturable reactor 42 thereby to decrease the voltage supplied to the field winding. Conversely, a decrease in the voltage across the field winding will have the efiect of decreasing the conductivity of the electric valve 6|, thereby increasing the conductivity of the valve 60 and increasing the field voltage.

The purpose of the adjustable resistor 62 which is connected between the negative terminal of the field weakening potentiometer l9 and the negative bus is to prevent the reduction of the field voltage to zero when the slider l9a of the field weakening potentiometer is at the maximum speed position, and to insure that it will be reduced to a preselected minimum value corresponding to the field weakening range of the motor with which the equipment is to be used.

If it should be desirable to use fieldcurrent rather than field voltage as the signal voltage, this may be done by substituting a voltage proportional to current for that portionoi the field *voltage which is impressed on the grid circuit of the .electric valve 6|.

- The armature speed control potentiometer I8 and the field weakening control potentiometer I! are preferably combined on a common shaft with the resistance portions i8 and I9 arranged circumierentially and with the sliders so oriented that with the speed control knobin the zero position, which may be assumed to be the full counterclockwise position, the sliders I81: and lab will also be in the zero position. Thus when th speed control knob is turned in a*clockwise direction from the zero position, the slider lBa oi the armature voltage potentiometer taps oil an increasing portion of the reference voltage of valve 54, but the slider Illa of the field weakening potentiometer slides along the contact strip I91) and therefore taps off the full voltage of tube II which corresponds to a condition of full field excitation. However, when the control knob passes maximum voltage can be impressed 1 position I, the slider or the armature speed control potentiometer slides on -the contact strip lib so that it taps ofl the full voltage or tube It corresponding to full armature excitation, but the slider Isa oi the field weakening potentioniv eter begins to tap oil decreasing portions of the voltage across the tube 54 so that the field will be progressively weakened as; the control knob is moved in a clockwise direction. 7 In the extreme clockwise position, which is point 8 on the dial, the armature thyratrons I4 and I! are supplying rated voltage to the armature and the field thyratrons i3 and it are supplying the.

minimum voltage to the field winding Illa. There-'- iore, the motor will be rotating at maximum 1 speed.

For the purpose of limiting the armature current to a maximum permissible value, means are provided for comparing a signal voltage derived from the anode current of the armature thyratrons with a reference voltage and utilizing the difierence of these signal and reference voltages to control both the armature and field thyratrons in such a manner as to limit the armature current to the desired value. These means are illustrated as comprising the anode current transformer 3|, the bi-phas rectifying electric valve 63, and control amplifier valves 64 and 65. As shown, the two primary windings 3 la and 3iboi the anode current transformer 3! are connected inseries with the anode circuits of eachoi the armature thyratron valves and, this transformer is polarized in such a manner that when one of the armature thyratrons conducts, the flux in the core is in one direction and when the other thyra tron conducts, the flux is reversed. As a result, an alternating current voltage is induced across the secondary winding 3lc and the magnitude of this induced voltage will be determined by the resistance load connected to the secondary and by the turn ratio between primary and secondary windings. A non-linear resistance 68 which is similarto the non-linear resistances 2| and 22 is connected across-a portion of the secondary winding to absorb the voltage surges which are induced in the secondary winding when the load current changes abruptly. A resistor 61 which is connected across another portion of the secondary winding determines the magnitude of the alternating current voltage that will be developed for a given direct current in the armature circuit. This alternating current voltage is'rectified by the electric valve 63 and appears as a direct voltage across a voltage divider comprising resistors 68, 69 and 59. l'

The electric valves M and are provided with anodes 64a and 65a, cathodes {54b and 55b, and grids 64c and 650, respectively. The anode 64a of valve 84 is connected to the junction point between the sections 51a and 51b of the voltage divider to which the grid 550 of valve 55 is con-.-

nected, and the cathode 64b of valve 84 is connected to the point 53a to which the cathode 55b of valve 55 is connected. The anode 65a of valve 65 is connected to the conductor 52 and the oath-' ode his connected to the grid 800 of the field control valve illwhich,,as shown, is connected to the junction point between the resistance sec tions 10b and 10c of a voltage divider comprising resistances 10a, 10b andloc connected across the direct current control system busses 52 and 20. .The ratio of the resistance section or this voltage divider is such that the voltage of the cathode" 65b is slightly-more negative than the voltage of cathode "b.

When small amounts of current flow'in the armature circuit, the voltage developed across the secondary of the current transformer and the voltage rectified by the valve 53 will be so small that the portion which is utilized as a signal voltage, 1. e., the voltage between the slider 88a and the negative bus 20, will be substantially less than the voltage across the tube 54, and consequently, the grids of the valves 64 and 65 will be very much negative with respect to their cathodes which are connected to the conductor 58a and to the junction point between resistance sections b and 10c, respectively. An increase of armature current will cause .the voltage across the secondary of the current transformer to increase, and

the voltage rectified by the valve 53 will increase correspondingly so that the voltage between the slider 68a and the negativ bus will ultimately reach a value approximately equal to the voltage across the valve 54 and the negative grid voltage of valves 64 and 55 will be reduced to the value at which these valves begin to conduct current.

When valve 54 conducts current, it has the same effect as if valve 58 were conducting current, which ls to decrease the current conducted by valve 55 and thereby decrease the'saturation of the armature saturable reactor 40 and decrease the voltage supplied to the armature.

The operation of valve 55 is slightly different in that its cathode is connected to the grid of a valve 50 instead of to the cathode, and its anode is connected through a resistor to the positive bus 52. When the grid of valve 65 is made sufficiently less negative to cause current to flow in the anode circuit, the effect of current flow through valve 55 is to raise the voltage of the junction point of resistors 10b, and 100 to which the grid 500 of valve 60 is connected. This has 'the effect of increasing the conductivity of valve 50 with the result that the field of the motor ID will be strengthened if it has been in a field weakened condition! Since the cathode 65b of valve 65 is more negative than the cathode 84b of valve 84, the valve 60 willbe controlled slightly ahead of valve 55, with the result that the field will be strengthened before the armature voltage is decreased.

In a typical installation, the current limit control may be set, by adjustment of the slider 58a, for 150 per cent normal full load current. In this case, the current limit control will be inactive up to approximately 150 per cent full load current because that portion of the direct current voltage proportional to armature current a between the slider 68:; and the negative bus 20 is less than the voltage drop across the valve 54 'with which it is being compared. Therefore,

the grids of valves 54 and 55 will be very much negative with respect to their cathodes and these valves will be cut off. To allow for a range of adjustment, the resistor 58 is made in the form of a potentiometer and the circuit in which it is included is so designed that with the slider at the Junction point between resistors 68 and 65, maximum voltage must be produced across the secondary of the current transformer 3| before the voltage between the slider 68 and the negative bus 20 will be sufficiently near equality with the voltage drop across the valve 54 to effect control of the valves 64 and 55. With the slider 68a at the opposite end of the resistor, only the minimum value of voltage need be developed across the secondary of the current transformer to control the valves 54 and 55.

For the purpose of accurately maintaining the speed of the motor at the level which is preset upon the speed controlling potentiometer l8, l9, means are provided for compensating for the RI drop in the armature circuit. In effect, these means subtract an increment of voltage from the armature terminal voltage so that the resultant voltage which is used as a signal of speed is approximately equal to the countervoltage of the motor. That is to say, to hold constant armature countervoltage is to hold constant speed and this is possible when the armature terminal voltage is increased by an amount equal to the RI drop of the armature circuit.

The potentiometer 59 serves to subtract the increment of voltage from the terminal voltage of the motor; The slider 59a of this potentiometer is connected to the negative end of the armatur'e voltage dividing resistance network resistors 58a and 58b. The voltage across resistor 59 is proportional to the armature current. The end of resistor 59 which is connected to the negative bus 20 is positive, and the end connected to the midtap of the secondary winding Me of the current transformer is negative so that the slider is always negative with respect to the negative bus 20, and this negative voltage is added at the lower end of resistor 58b. The circuit functions in such a manner that as the armature current increases, the voltage of the lower end of resistor 58b is made more negative with respect to the negative negative by the voltage drop between the negative bus 20 and the slider 59a. Thus, an increase of current through potentiometer 59 has a tendency to make the grid c negative, and that tendency causes an increased armature voltage to re-establish the voltage of the grid at its former level. Since the voltage drop across the resistance 59 is proportional to armature current and since the RI drop is also proportional to armature current, it is possible by adjustment of the slider 59a to select the voltage drop which will cause the armature terminal voltage to be increased by an amount equal to the internal RI drop of the motor plus the RI drop of all connecting leads.

When a motor is operated at rated armature voltage and at maximum rated field current and is carrying rated full load, it is said to be operating at base speed. If the motor In is being operated in the field weakened range, e. g. three times base speed, and if then the speed controlling potentiometer IB, i9 is suddenly changed to a position of lower speed, e. g., one-half base speed, the control would function to decrease the voltage supplied by the armature thyratrons l4 and I5 and to increase the voltage supplied to the field winding by the field thyratrons i3 and I5, as a result of the action of electric valves 55 and 56 and 69 and BI. With the motor running at three times base speed and full field applied as quickly as the time-constant of the magnetic circuit will permit, the tendency is for the armature countervoltage to increase to a value. which is approximately three times full terminal voltage, i. e., 750 volts in the case of -a 250-volt motor. To prevent such an undesirable increase in the armature voltage, an additional electric valve II is provided. This valve is provided with an anode Ila, a cathode Ilb, and a grid He. The anode lie of this valve is connected to the Junction point of the resistors 10a and 10b of the voltage divider to which the grid "c of valve 60, is connected, and the cathode Nb of valve II is connected to the conductor "a. The grid 1 Ic of valve II is connected to the junction point 12a of two resistors I2 and II which constitute 'a voltage divider connected across the armature of motor III. As thus connected, this valve II measures a fixed portion of the armature voltage, i. e., the portion between the junction point 12a and the negative-bus 20, and when this portion exceeds the voltage drop as aur To place the system in sistorllb which constitutes one element of the voltage divider comprising the resistors Ila, 51b,

and tie to which the grid 550 of electric valve 85 is connected. with the foregoing understanding of the elements and their organization in the completed sumcient to provide for. initial heating of the across the reference voltage valve 5|, the valve 1 II becomes conducting, thereby increasing the voltage drop across the resistor a and decreasing the voltage of the grid We of valve 80 to preventthe saturation of the saturable reactor l2 which controls the field thyratrons I3 and IS. The action of valve 1| upon valve 60 is very similar to the action of valve 8i except that the valve II receives its voltage signal from the armature circuit, whereas the valve 8i receives its voltage signal from the field circuit. Thus, valve 'II acts as a cross connection between the armature circuit and the field circuit to prevent the field thyratrons I3 and it from increasing the field strength to a point at which the armature voltage would exceed a safe limiting value. In practice; this limit of armature voltage would be set at some value between 300 and 850 volts for a motor whose operating voltage is 250 volts. At voltages below the voltage limit setting the grid of valve II is rendered inactive because its grid voltage is very much negative owing to the fact that the voltage between the junction point He and the negative bus is very much less than the voltage drop across the reference voltage valve 54.

Owing to the loss of cooling by lack of windage, a motor cannot conduct the same armature current at the very low speeds which are obtained by reduced armature voltage that it can conduct at the higher speed ranges for which it is designed. To this end, the speed control potentiometer I8, I9 may be mechanically coupled to the current limit control potentiometer 88 so that as the sped control potentiometer I8, I9 is turned toward the lower speed position, the'current limit setting its correspondingly reduced. This prevents the possibility of overloading the motor excessively at very low speeds.

In order to prevent the motor coasting for a long time at high speed when the speed controlling potentiometer is moved from a high speed position to a low speed position, means are provided for dynamically braking the motor from the higher speed lever to the lower speed level. These means are illustrated as comprising an electric valve I3 and a contactor ll conbus 52 through the operating coil of the contactor I I4 to the anode 13a and from the cathode lib to the conductor 53a. The grid 13c of valve I3 is connected to an intermediate point on the refor completing a dynamic brakthe resistor 15 in parallel cathodes of the electric valves, the time delay relay 29 closes its normally open contacts 29a to complete the field circuit from the cathodes of the field thyratrons I3 and ,IG through the operating coil of field protective relay 28 and the field winding IIlb to the negative bus 20.

Assuming that it is desired to operate the motor. at a speed above base speed within the field weakened range, the knob of the speed controlling potentiometer is moved until the sliders Ito and I9a are at a position between- 5 and 8 which corresponds to the desired operating speed. As a result, the field thyratrons I3 and It will supply a current to the field winding "lb of the proper value for operation of the motor at the preselected speed. Since the operating coil of relay 28 is energized by this field current, relay 28 has picked up and closed its normally open contacts 28a. v a

To start the motor in the forward direction, the forward push button switch 45 is depressed to bridge its normally open contacts in thereby to complete an energizing circuit for the operating coil of the forward contactor 31. This circuit is traced from the low voltage tap I2c through conductor 11, operating coil of con- ,tactor 31, contacts 450. of the forward push butand 31a and opens its normally closed contact 31!. Main contacts 314: and 31b in closing complete the armature circuit from the cathodes of the armature thyratrons by conductors l2 and 33 to the terminal 34 and through conductor 35 and the coil 38a of the overload: relay, main contacts 31a, armature Illa, and main contacts 31b to the negative bus 20. Since the slider Ila is on the maximum voltage position,-

the valve 55, if its anode-cathode circuit were complete at this point in the operation, would be fully conducting and the armature thyratrons I4 and I5 would tend to supply maximum voltage. However, the anode circuit of valve 5! is open at the contacts 30c of the anode contactor 30. Consequently, the phase of the grid voltage of the armature thyratrons is fully retarded so that when the contacts 300, and 30b of the anode contactor are subsequently closed. the output of the armature thyratrons starts from a minimum value. If the anode circuit of the valve 55 were completed and the grid voltage of the armature thyratrons fully advanced push button it, normally closed contacts of stop button 41, and contacts 28a and 36b to the opposite low voltage tap in. Contacts lie in closing complete the energizing circuit for the operating'coil of anode contactor 30 from conductor ll which is connected to the low voltage tap lie, through the operating coil of contactor l and contacts 31c and thence, by the circuit previously traced through the contacts 01' the stop push button to the opposite low voltage tap l2b.

In response to energization, the anode contactor 30 closes its normally open contacts 36a, 30b, 30c, and Md. The contacts 30a and 40b in closing complete the anode circuits of the armature thyratrons I4 and I5, and contacts "c in closing complete the direct current saturating circuit of the armature saturable reactor 40 through the valve 55, and thus armature current is allowed to build up at a time .rate determined by the inductance of the direct current winding of the armature saturable reactor.

Prior to the opening of the normally closed contact 31! of the forward contactor, the Junetion point between the resistors 68 and I9 is connected by means of conductor 18, contact 31], and conductor 19 to the positive control voltage bus 52. As a result, a positive voltage is applied to the grids 84c and 650 of the armature current limiting control valves 64 and 65 so that the circuit is given the impression of operating under conditions of excessive armature current, and the control valves 84 and 85 tend to desaturate the armature saturable reactor 40 and to saturate the field saturable reactor 42. when the forward contactor close its main contacts and opens contact 31!, this false signal circuit is interrupted and after the anode contactor contacts 30a and 30b have closed as a result of the closing of the forward contactor. the armature current is allowed to build up at a rate determined by the inductance of the d rect current winding of the armature saturable reactor. If desired, either the false signal circuit II, II or the interlock contact 300 on the anode contactor can be omitted. Contacts 31c of the forward contactor in closing complete a holding circuit in parallel with the contacts 45a of the forward push button which'may now be released.

As a result of the completion of the armature circuit, the motor begins to accelerate to a speed determined by the setting of the speed control potentiometer. During acceleration, before the armature countervoltage has built up to a value corresponding to the preset speed which it is desired to maintain, the phase of the grid voltage of the armature thyratrons tends to be fully advanced and hence, the armature thyratrons tend to supply a current to the armature which is many times full load value. However, the current limitin; control acting through valves 04 and dev sistor ll.

creases the output of the armature thyratrons to the value determined by the setting of the slider oi the current limiting potentiometer 88. If during acceleration, the field control has been set for a speed in the field weakening range, as assumed, then the current limit control acting through valve 65 will tend to maintain full field until the armature current tends to fall below the preset limiting value. Thus, during acceleration to a preset speed within the field weakening range, the armature voltage is first allowed to build up at a rate determined by the load on the motor and by the value of the armature current which has been set upon the current limiting potentiometer i8 until full armature voltage is reached. At this point, there is a tendency for the armature current to tend to decrease. However, this tendency will make the grid voltage of the valve more negative thereby decreasing the conductivity of the valve and making the voltage of the grid "c of valve 60 more negative with the result that the field thyratrons l3 and It will supply less current to the field. This results in maintaining the armature current constant until that preset field weakened speed is reached at which point the armature current will drop to that value whichis necessary to drive the load. Since the system operates to maintain the maximum permissible value of armature current during the acceleration, the

load is accelerated to the preselected speed in the minimum possible time consistent with the armsture current limit at which the control is set to operate.

If, while the motor is operating at the preselected high speed assumed, it should be desired to reduce the speed to a substantially lower value, the speed controlling potentiometeris moved to a lower speed position. Assuming that the new speed is less than the base speed of the motor, the control will operate to strengthen the held of the motor and to decrease the voltage supp ied to the armature. With some types of load, the motor would tend to coast for some time at the higher speed after which the speed would gradually be reduced to the new preset value. Such a delay in changing from a higher speed level to a lower speed level is frequently undesirable, and in the present control this delay is eliminated by means of the dynamic braking circuit which is under the control of the electric valve 13. This circuit functions in the following manner: When the electric valve 55 is conducting and controlling the current through the armature saturable reactor 40, the grid of valve 13 is more positive than the grid of valve 55 owing to the bias adjustment shown in the drawing. Consequently, the grid of valve 13 will be sufficiently positive to cause valve 13 to conduct sufficient current through the operating winding of dynamic braking contactor 14 to maintain the contact "a open. When the speed controlling rheostat l8, i9 is adjusted for a lower speed, the grid of valve 55 is made more negative and the grid of valve 13 becomes sufficiently negative to reduce the current through valve 13 to the value at which the dynamic braking contactor 14 drops out and closes its contact Ila to complete a dynamic braking circuit for the armature Ila through the dynamic braking re- As a result, a large braking torque is developed and the speed of the motor is rapidly reduced to the new value. As the speed of the motor reaches the new value, the grid voltage of valve 58 is reduced to a value corresponding to the new setting of slider i8a with the result that valve ll conducts less current so that the grid voltages gridv voltage of. valve ll increases the current conducted by the valve I3 to the value at which contaotor 14 picks up and opens its contacts 14a to I supplied to said motor to accelerate said motor interrupt the dynamic braking circuit for the armature. I

To stop the motor, the stop pushbutton 41 is depressed to open its normally closed contacts. The opening of the contacts of stop push button 41 interrupts the circuit of the alternating current winding of the armature saturable reactor 40 which was previously traced through the contacts of the stop push button. As a result, the phase of the grid voltage of the armature thyratrons H and II is retarded, thereby rendering the armature thyratrons l4 and I5 non-conducting so that when the main contacts of the forward contactor 3.1 and the contactsjoa and b of the anode contactor subsequently open as a result of the deenergization of their operating coils, the contacts of the directional contactor and anode contactor will not be required to'interrupt any current. As a result, the construction of the directional cone tactors and the anode contactor can be much lighter than would otherwise be required." In fact, the directional contactors and anode contactor may be of a very light relay construction. The anode contactor in dropping out closes its normally closed contacts the to complete a dynamic braking cirouitthrough the dynamic braking resistor 18 in parallelwith the armature. As

a result, a dynamic braking torque is developed which rapidly brakes the motor torest.

It will be noted that the circuits of the alternating current windings 40a and 42a. of both the armature and field saturable reactors l0 and- 42 pass through the. contacts 36b of the overload protective relay 38. Consequently, it while the motor is operating, the overload relay be interrupted with the result that the phase .of the grid voltages of both the field thyratrons 13 and I8 and the armature thyratrons and il would be retarded and the armature and field currents reduced substantially to zero. 1

Since the energizing circuits for the, operating coils of the directional contactors and the anode. contactor also pass through, the contacts 36b of the overload relay, the opening of contacts 361; also effects dropout of the directional contactors and anode contactor.

Although in accordance with the provisions of the patent statutes this invention is described as embodied in concrete form and the principlethe art without departing from'the true spirit of this invention or from the scope of the annexed claims.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. Amotor starter comprising in combination,

electric valve means for supplying-a voltage to said motor, means for presetting an operating speed for said motor comprising means for producing a reference voltage corresponding to the desired speed, means for producing a voltage proportional to the speed of said motor,-means for rendering said electric valve means con- 36 should open its-contacts owing to an overload, the alternating current circuits of both reactors would.

of valves II and 13 increase. The increase in the ducting, an auxiliary electriovalve responsive to the difference of said reference voltage and said voltage proportional'to speed for controlling said electric valve means to increase the voltage to said desired preset speed, and means responsive to the armature current of said motor for controlling said electric valve means to'limlt the armature current of said motor to a predeter- 10 mined value.

2. A motor starter comprising in combination, means for presetting an operating speed for the motor comprising a source of substantially constant voltage and means for deriving a voltage" 15 therefrom corresponding to a desired operating speed, means for deriving a voltage from the armature of said motor corresponding to the speed of said motor, an electric valve having an anode, a cathode, and a control grid, connections for applying one of said derived voltages to said grid and the other to said cathode-to vary the condition of conductivity of said valv responsively to the difference between the spegd of said motor and said preset speed, and elec ric valve means responsive to the condition of conductivity of said valve for supplying a voltage to said motor to accelerate said motor to said preset speed, and means responsive to the armature current oiisaid motor for controlling said eleciiO trio valve means to limit the armature current of said motor to a predetermined value.

3; A motor starter comprising in combination, a -sourceoi. alternating voltage,'means for controlling the supply of current'to the motor comprising an electric valve provided with an anode,

a cathode,- and a control grid and having its anode connected to said source and having its cathode connected throughv said motor to said source, means for varying the conductivity; of

40 said valve to accelerate said motor,'a transformer,

, having its primary winding included in the con-'- nection between said source and said anode,

. means for rectifying the} secondary voltage of.

, said transformer, and means responsive to' said 45, rectified voltage for controlling said electric valve to limit thearmature current of said motorto a predetermined value.

4. A motor starter comprising a source of alternating voltage, a transformer having apri-- mary winding connected to said source and a secondary winding provided with a midtap, means for controlling the supply of current toa'motor comprising electric valve means having a pair of anodes a cathode and a control grid,

connections from the terminals of said secondary winding to said anodes and connections fromsaid cathode and midtap to a motor, means for controlling th voltage of said'grid to accelerate said motor, a control transformer having a pair of primary windings connected respectively in said connections to-said anodes, a full wave rectifier for rectifying the secondary voltage of said control transformer,- and means responsive to said rectified voltage for controlling said electrio valve to limit the armature current of'the motor to a predetermined value.

.5. In combination, an electric motor having an armature and a field winding, a first electric valve means for supplying a voltage to said armature, a second electric valve means for supplying current to said field winding, means for presetting an operating speed for said motor comprising a control member for said valve means movable through a range of positions to control said first valve means toincrease the voltage supplied to said armature and movable through a succeeding range of positions to control said second valve means to weaken the current; supplied to said field winding, and means responsive to armature current for controlling said first valve means to limit the current supplied to said armature to a predetermined value and for preventing said second valve means from weakening said field current until said armature current tends to decrease below said predetermined value.

6. In combination, an electric motor having an armature and a field winding, a first electric valve means for supplying a voltage to said armature, a second electric valve means for supplying current to said field winding, means for presetting an operating speed for said motor comprising a control member movable through a range of positions to prepare said first electric valve means for increasing the current supplied to said armature and movable through a succeeding range of positions to prepare said second valve means to weaken the current supplied to said field winding, a starting control device, means responsive to operation of said device for rendering said valve means conducting, and means responsive to the current supplied to said armature for controlling said first valve means to limit the current supplied to said armature to a predetermined value, and for controlling said second valve means to weaken the current supplied to said'field winding to maintain said armature current at said predetermined value until the speed of said motor increases to said predetermined value.

7. A starter for a motor having an armature and a field winding comprising in combination, means for presetting an operating speed for said motor comprising means for producing a first reference voltage corresponding to the terminal voltage of said motor at said operating speed, means for deriving from said motor a voltage dependent upon the speed of said motor, means for responding to the difierence of said voltages for increasing the supply of voltage to the armature of said motor, means for producing a second reference voltage corresponding to the voltage across said field winding at said operating speed, means for deriving a voltage from said field winding dependent upon the voltage across said winding, and means for responding to the difference of said second reference voltage and said voltage derived from said field winding for weakening the current supplied to said field winding.

8. A starter for a motor having an armature and a field winding comprising in combination, means for presetting an operating speed for the motor comprising means for producing a signal voltage corresponding to the terminal voltage of said motor at said operating speed, means for producing a reference voltage corresponding to the voltage across said field winding at said speed, means for deriving voltages from said motor and field winding, auxiliary electric valve means controlled by said reference voltages and derived voltages, and electric valve means controlled by said auxiliary electric valve means 'for supplying current to said armature and field winding.

9. A starter for an electric motor having an armature and a field winding comprising electric valve means for supplying current to the armature and field winding of said motor, means for presetting an operating speed for the motor comprising a control member 'movable through a range of positions to prepare said valve means for increasing the voltage supplied to said armature and decreasing the current supplied to said field winding, a start control device and means responsive to operation thereof for rendering said valve means conducting, and means responsive to the armature current of said motor for controlling the order of occurrence of said armature voltage increasing and field current weakening operations.

10. A starter for an electric motor having an armature and a field winding, a first electric valve means for supplying current to said armature and field winding, means for presetting an operating speed for the motor comprising means for producing a first reference voltage corresponding to the terminal voltage of said motor at said operating speed, means for deriving a voltage from said motor dependent upon the terminal voltage of said motor, an auxiliary electric valve having its conductivity controlled by the difference of said voltages, means for producing a second reference voltage corresponding to the field strength of said motor at said speed, means for deriving a voltage from the voltage across said field winding, a second auxiliary electric valve having its conductivity controlled by the difference of said second reference voltage and the voltage derived from said field winding, means responsive to the condition of conductivity of said auxiliary valves for controlling said electric valve means to increase the voltage supplied to said armature and weaken the current supplied to said field, additional electric valve means provided with a control grid, means for applying to said grid a voltage derived from said armature current for controlling the conductivity of said additional valve means, and means responsive to the condition of conductivity of said additional valve means for controlling said first electric valve means to limit the current supplied to said armature to a predetermined value and to weaken the current supplied to said field winding only after said armature current tends to decrease below said predetermined value.

11. A starter for a motor having an armature and a field winding comprising electric valve means for supplying current to said armature and field winding. 9. control member for said valve means continuously movable through a range of positions to control said valve means to increase the voltage supplied to said armature and through a succeeding range of positions to control said valve means to weaken the current supplied to said field winding, and means responsive to the armature voltage of said motor for controlling said valve means to limit the voltage of said motor to a predetermined value by controlling the excitation of said field winding when said control member is returned from a weakened field position to a strengthened field position.

12. A starter for a motor having an armature and a field winding comprising in combination, means for supplying current to said armature, electric valve means for supplying current to said field winding, said electric valve means being provided with a. control grid, means for controlling said electric valve means to weaken the current supplied to said field winding to accelerate said motor comprising an auxiliary electric valve provided with an anode, a cathode, and a control grid, a control member continuously movable through a range of positions and means controlled thereby for varying the grid to cathode voltage of said auxiliary valve, and means for controlling said electric valve means to limit the mined value when said control member is returned from a weakened field position to a strengthened. field position comprising a second auxiliary electric valve provided with an anode, a cathode, and a control grid, and meansior supplying to the cathode and grid of said second auxiliary valve a voltage derived from said motor and proportional to the counter-voltage thereof.

13. A starter for an electric motor having an armature and a field winding comprising in combination, means for presetting an operating speed for said motor comprising means for producing a reference voltage corresponding to the desired-speed of said motor, means for deriving from said motor a voltage proportional to the terminal voltage, an electric valve provided with an anode, a cathode, and a control grid, electrical connections for applying the difference oi said reference voltage and derived voltage to said grid and cathode to control the conductivity oi! said valve, electric valve means responsive to the condition of conductivity of said valve for supplying a voltage to the armature 01' said motor dependent upon said voltage difierence, and means responsive to the current supplied to said armature for modiiying the grid to cathode voltage of said valve thereby to compensate for internal voltage drop of said armature.

- 14. A starter for an electric motor having an armature and a field winding comprising in combination, means for presetting an operating speed for said motor comprising means for producing a reference voltage corresponding to a desired operating speed of said motor. a first circuit deriving a voltage from the motor proportional to the terminal voltage of said motor, an electric valve provided with an anode, a cathode, and a control grid, electrical connections for applying the diflerence of said reference voltage and derived-voltage to the grid and cathode of said valve to control its conductivity, a source of alternating voltage, electric valve means responsive to the condition of conductivity oi said valve for supplying a voltage to said armature dependent on said voltage difierence, said electric valve means being provided with an anode, a cathode,

armature voltage of said motor to a predeter I and a control grid, supply connections from said source to the anode-cathode circuit of said electric valve means, means comprising a current transformer having its primary winding included in the connections between said source and the anode of said electric valve means and a rectifier in the secondary circuit or said transformer for producing a rectified voltage proportional to the current supplied to said armature, and connections from said secondary circuit to said first circuit for modifying the grid to cathode voltage of said electric valve to control said electric valve means to compensate the voltage supplied to said armature for the internal voltage drop of said armature.

15. In combination, an electric motor, unidirectional electric valve means for controlling the supply of current from a source to the armature of said motor, means for controlling the speed of said motor comprising a device movable through a range of positions corresponding to the speed range of said motor, means responsive to movement of said device from a speed controlling position to a lower speed position for completing a dynamic braking circuit for said motor for rapidly reducing the speed ot said motor to a value corresponding-to said-lower speed position, and means responsive to said lower speed of said motor for interrupting said dynamic braking to provide for continued operation of said motor'at said lower speed.

16. In combination, an electric motor, unidirectional electric valve means for controlling the supply of current from a source to said motor, means for varying the speed of said motor throughout a substantial range of values, an

electric valve connected to be responsive to a speed reducing operation of said speed controlmeans for controlling the completion of a dynamic braking circuit for said motor to effect a reduction of the speed to a lower value corresponding to the extent of operation of said speed control means and responsive to operation .of said motor at said lower value for interrupting said dynamic braking circuit.

-. ELMO E. MOYER.

ORRIN W. LIVINGSTON. HENRY H. LEIGH.

1 Patent No. 2,312,117.

CERTIFICATE OF, CORRECTION.

February 25, 19115. m'wE. noYER, E'r-AL'.

*It i: hereby certifiedithat error appears in the printed epecificatiofi of the above numbered patent requir'ipg correction a: followsi Page 3, first column, 11I1'e-."5,-.for 'Ytfibe's read --tube--; and second columnjline 75 forl'poei tion'. reed -pert 1 n--, b nge 6, first colfimn, 1121055, for "speed" read --sp'ee d--; 1111a 55, for "its" read --1e--; line 65, for lever' read level--; and that the said Letters; Patent shoulc} be read with this correctien therein" that the same-may conform V to the record of the casein the Petentbfiicet Signed end aeeled :this 6th d'ay bf April, A. D. 191 5.

7 Henry Van Arsdale, (Seal) Acting Commissioner of Patents.

. Patent No. 2,312,117.

CERTIFICATE OF, comcnon.

4 February 25, 1%}- e mmE. HOYER, E'r-AL'.

is hereby certifledithat error appears in t'he printed epeciiicatior of the above numbered patent requir'ipg correction as followsi Page 5, first column, 11nd. 55, for 'Ytube's" read --tube--; and aecend columnfline 75-,

-tor "posi i;ion" read -po rt iqn--, piige 6, first colfimn, line ,55 for "sped" reed --epeed--; lihe 55, for 'ita' read --ie-; line 65, for "lever" read -level--; and that the said Letters- Patent should be read with this correctien therein" thet the same may conform to the record of the case in the Patent of iicet signed and aeeled this 6th day 'or April, A. n'. 1915,

.Henry Van Arsdale, (Seal) Acting Commissioner of Petents. 

